FIG. 4 shows a circuit diagram for a conventional superhigh pressure generator system employed in a water-jet type cutting apparatus (Japanese Patent Application Laid-open Publication No. 63-39799). The superhigh pressure generator system comprises a booster 61 including a double acting oil hydraulic cylinder 62 having a piston P and plungers P.sub.1, P.sub.2 arranged at opposite sides thereof and fitted respectively in water-pressurizing plunger chambers C.sub.3, C.sub.4, and ports at distal ends of the plunger chambers which are connected in parallel to a water supply line 66 from a water supply pump 65 via suction check valves 63, 64, the ports being also connected in parallel via discharge check valves 67, 68 to a superhigh pressure water discharge line 69 provided sequentially with an accumulator 70, a nozzle on-off valve 71, and a jet nozzle 72. A two-position directional control valve 74 for switching the reciprocating motion of the piston is provided between the respective ports at opposite ends of a cylinder chamber of the oil hydraulic cylinder 62 and an oil hydraulic pump 73. Air nozzles 77, 78 are fixed adjacent the jet nozzle 72 and in slightly spaced apart therefrom in the directions of movement (designated by arrows X, Y) of a moving carriage 75 on which is carried a workpiece 76, the air nozzles being connected to a pneumatic power source 81 via on-off valves 79, 80. Relief valves 85, 86 are respectively disposed between the water supply line 66 and a water tank 82 and between a main line 83 for the oil hydraulic pump 73 and an oil tank 84.
When the hydraulic pump 73 is actuated with the two-position directional control valve 74 set to assume a symbol position V.sub.1, hydraulic oil is supplied to a cylinder chamber C.sub.1 and the hydraulic oil in a cylinder chamber C.sub.2 is discharged into the oil tank 84, so that the piston P shifts to the right side and the water in the plunger chamber C.sub.4 is pressurized by the plunger P.sub.2 to be boosted in proportion to the ratio of sectional area of the piston P to the plunger P.sub.2. The water which is boosted by the booster 61 to superhigh pressure (e. g., 3000 kgf/cm.sup.2) is ejected from the jet nozzle 72 toward the workpiece 76 after passing through the check valve 68, accumulator 70, and the nozzle on-off valve 71 at symbol position V.sub.11. Whilst, water from the wager supply pump 65 is sucked via the check valve 63 into the plunger chamber C.sub.3, as the pressure therein turns negative as a result of the shifting of the piston P to the right.
Subsequently, when the two-position directional control valve 74 is switched to symbol position V.sub.2, the hydraulic oil from the hydraulic pump 73 is supplied to the cylinder chamber C.sub.2 and the piston P is shifted to the left, so that the water in the plunger chamber C.sub.3 is pressurized by the plunger P.sub.1. Thus, the water boosted to superhigh pressure is similarly ejected toward the workpiece 76 via the check valve 67 and other associated members. Whilst, water from the water supply pump 65 is sucked into the plunger chamber C.sub.4 which is now under negative pressure.
Since the superhigh pressure generator system will pressurize the water within the plunger chambers C.sub.3, C.sub.4 to a superhigh pressure of as high as 3000 kgf/cm.sup.2, it is likely that seals fitted on the plungers P.sub.1, P.sub.2 which slide within the plunger chambers will become abrasively damaged during prolonged use. In order to insure continued operation of the generator system in the event of such an occurrence, it is necessary to provide a reserve booster which can be switchably connected to the oil and water hydraulic circuit of FIG. 4.
However, since the superhigh pressure generator system is a booster having superhigh pressure plunger chambers at opposite sides of the double rod-type hydraulic cylinders 62, if the seal on one side becomes abrasively damaged, the booster 61 as a whole can no longer be used as such. For this reason, the generator system also includes one reserve booster of an identical construction. Then, with the prior art reserve booster which comprises a double rod-type cylinder having a pair of plunger chambers for the same work as could be performed by a booster having one plunger chamber, there is caused a problem that such an arrangement results not only in increased cost of manufacture, but also in increased equipment size.